Microwave tube with resonant discharge gap



United States Patent Qfifice Patented July 29, 1958 MICROWAVE TUBE WITHRESONANT DISCHARGE GAP Louis D. Smullin, Water-town, Mass, assignor toSylvania Electric Products lnc.,a corporation of MassachusettsApplication January 6, 1953, Serial N 0. 329,853

6 Claims. (Cl. 315-41) The present invention relates to waveguidestructures and in particular to transmit-receive tubes and the likehaving discharge gap electrodes.

For some years, transmit-receive switch tubes or duplexers have beenused in radars where a common antenna system is utilized both by a pulsetransmitter and by a sensitive receiver, the transmit-receive switchoperating to short-circuit the wave guide communicating to the receiverfrom the antenna during the high-level pulses emitted by thetransmitter. These tubes include a pair of discharge gap electrodes, inthe customary design, and have been variously assembled in cavityresonators, and in various waveguide configurations. At the time of thehigh-level micro-wave pulse, a discharge develops across the gapelectrodes, resulting in an ionized discharge in the tube, which isgas-filled, and the high level energy then is reflected from thedelicate receiver. Weak signals received at the antenna are transmittedthrough the transmit-receive tube which is not ionized by such signals.Similar tubes having discharge gap structures include the attenuatortube that is eflective to limit the sensitivity of a receiver during apredetermined time after each transmitter pulse, and combined attenuatorand transmit receive tubes. The present invention will be recognized ashaving application to a variety of tubes as indicated, but is hereinexplained in connection with a broad-band transmit receive tube to whichthe invention is particularly applicable.

I The broad-band type of transmit-receive tube usually includes a lengthof wave guide with multiple discharge gaps disposed across thewaveguide. The gap electrodes, by virtue of their proximity to eachother, introduce a capacitance which would raise complex externalimpedance matching problems if the capacitance of each gap were nottaken into account within the transmit-receive tube structure. Inrectangular waveguide transmit-receive tubes with the discharge gapelectrodes extending toward each other from the broad top and bottomwalls of the wave guide, it has been customary practice to providebaflles on both sides of the gap electrodes, formed as blades extendinginward through slots in the narrow side walls and from top to bottom ofthe wave guide. Slots have been cut in the waveguide, and then theblades have been inserted and brazed in place. This construction isshown, for example, in Patent No. 2,611,109 issued September 16, 1952,to E. T. Casellini.

Broad-band transmit-receive tubes customarily include two or moredischarge gaps, and for each such gap, a pair of bafiles has beenrequired. This procedure is quite costly and uncertain because of theextensive brazing operations involved. A number of tubes may leak out ofa large production run of tubes, so that partly completed but leakytubes would be rejected and a corresponding burden of inspection iscreated. Transmit-receive tubes, it should be understood, arehermetically sealed and contain a particular mixture of ionizing andquenching gases, generally at a small fraction of atmospheric pressure.Any leakage thatwould change the composition of the gas or its pressureshould therefore be avoided.

In applying the present invention, a pair of discharge gap electrodes isassociated with a pair of posts. The discharge gaps which terminate ashort distance from each other represent capacitance and the postsrepresent inductance, the gap electrodes and the posts togetherconstituting a resonant circuit. Accordingly any tendency of the gapelectrodes to introduce reflections as -a localized capacitive reactanceis neutralized by the posts as inductive reactances at that location inthe wave guide. The positions, shapes and the cross-section of the postsinfluence their inductive effect. In transmit-receive tubes where anumber of discharge gaps are distributed one-quarter-wave length awayfrom each other at the mean frequency of the band for which the deviceis intended, each pair of discharge electrodes is associated with a pairof posts.

The posts or wires advantageously are disposed relatively close to thedischarge gap electrodes. In this position they extend across thewaveguide in the same region Where the discharge gap electrodesthemselves are mounted, and are accordingly susceptible to. a minimum ofbrazing diificulties, in respect to any tendency to develop leaks due toimperfect brazing, and are most readily assembled in place.

This invention provides a new resonant structure, involving a pair ofdischarge gap electrodes resonated by a pair of posts. This structurewhen incorporated in a rectangular waveguide extending from the opposedbroad walls of the guide vastly simplifies fabrication oftransmit-receive' tubes and the like, and attains further advantagesas'will appear.

The nature of the invention and its various features of novelty andobjects will be better appreciated from the following detaileddisclosure of an illustrative embodiment shown in the accompanyingdrawings. In the drawings:

Fig. l is a longitudinal cross-sectional view of an illustrativetransmit-receive tube along the line 11 of Fig. 2;

Fig. 2 is a cross-sectional view of the transmit-receive device of Fig.1 along the line 2-2 in Fig. 1; and

Fig. 3is' a lateral view of a discharge gap and post assembly as a unitin readiness for mounting as in a waveguide to constitute a transmitreceive switch tube like that in Figs. 1 and 2.

Referring noW to Figs. 1 and 2 showing an illustrative embodiment of theinvention, a length of rectangular metal waveguide 10 is shown having apair of end flanges 12, 14 provided for joining the tube to otherstructures in the waveguide system in which the tube is to be used. Onepair of discharge gap electrodes 18 is disposed be tween the broadtopwall 20 and the broad bottom wall 22 of the waveguide, approximatelyone-quarter wavelength away from window 24 formed as the closure at oneend of the waveguide. That window has a small opening covered by glassand is designed as a component resonant at the center frequency of theband for which the tube is to .be used.

A second pair of discharge gap electrodes 26, 28 is separated from thefirst pair 18 by a quarter-wave length, and a second window 30, closingthe opposite end of guide 10, is separated from electrodes 26, 28similarly by a quarter-wave length. Electrodes 18 are like electrode 26inconstruction.

Associated with each pair of gap electrodes 18 and 26, 28 there is apair of metal posts or wires 32 which, extending from the broad walls20, 220i the rectangular waveguide, introduce inductance which resonateswith the capacitive effect of the discharge gap electrodes, when Ioperated in the usual mode. In this mode, a signal potential isdeveloped between the broad walls of the waveguide and hence between thegap electrodes.

Discharge gap electrode 28 is hollowed and contains a keep-aliveelectrode 34 that is supported by a glass insulator 36 and has anexternal terminal 38 to which a voltage may he applied relative toelectrode 28. Features of this construction are more fully disclosed andclaimed in copending application Serial No. 237,258, filed July 17,1951, by Paul E. Gates, now Patent No. 2,740,186. The limited glowdischarge produced at the tip of the keep-alive electrode by the appliedpotential makes available a small volume of ionized gas for facilitating prompt response of the transmit-receive tube to the high levelmicro-wave bursts of transmitter energy which are periodically impressedon the tube, and which are reflected when the ionized breakdown occursand thus are not transmitted through the transmit-receive tube to thereceiver. Between these bursts, the tube is supposed to transmitlow-level signals and in this function is to act as nearly like an inertlength of waveguide as possible, and therefore should contain a bareminimum of localized impedances. This is realized for a limited band offrequencies, by the multiple quarter-wave spaced resonators.

Electrode 26 and one of the electrodes 18 is formed with afemale-threaded bore 40 which is externally accessible for tuningadjustment. By inserting a threaded tool and deforming the base flangeof the electrode, it is possible to shift its position laterally andendwise toward and away from the opposed discharge gap electrodes 28, tothat limited extent which may be necessary to perfect the tuning. Whenproperly tuned, discharge electrodes 26 and 28 coact with posts 32 toresonate at the center frequency of the band for which the device isdesigned.

Posts 32 as resonating elements contrast in many ways with the baffiesor blades previously used in this type of tube. Such blades could not bereadily adjusted. Once they were inserted and brazed in place allpractical tuning adjustments had to be made by deforming the support ofa gap electrode and if this was insufiicient the device would berejected. Those blades, brazed in slots in the waveguide, involveserious possibility of leakage developing, particularly where a largenumber of blades are required in a number of gaps.

As seen in Fig. 2, the posts 32 terminate in the same region as thatused for sealing the gap electrodes themselves and accordingly minimizethe extent of the brazing to be done, and this correspondingly minimizesthe occurrence of leaky tubes. Additionally, the posts can be deformedas by lateral deflection before the windows are sealed in place shouldit become necessary to adjust their inductive effects. This may beoccasioned either because the tuning range of the gap electrode may beinadequate or where it is desired to maintain a predetermined spacingbetween the gap electrodes.

The tub of Fig. 2 may be assembled variously. For example, after the topand bottom walls 20 and 22 have been formed with recessed seats for thegap electrodes, they may be drilled with aligned holes to receive posts32. The electrodes may then be assembled as illustrated, with seatedflanges or bases 48, 50 of the conical electrodes 26, 28, and while heldwith brazing preforms in a suitable brazing jig in a hydrogen furnace orthe like the parts are finally united. After the windows 24 and 30 aresimilarly brazed in place, the device is a hermetically sealed unit. Itis exhausted and then filled at a low pressure with a suitable gas, asfor example, argon and water vapor at equal partial pressures for atotal pressure of 10 mm. of mercury.

The tuning adjustment may be effected by deforming the posts 32 or ifthey should be unintentionally deformed, their configuration may becorrected before assembly of the windows; and the final tuningadjustment can be effected as heretofore by adjusting electrode 26.

As seen in Fig. 2, the flanges of electrodes 26 and 28 cross the ends ofposts 32; and when the flanges are brazed in place, there need be nospecial concern for hermetic sealing of the post ends.

The structure of Figs. 1 and 2 may be modified somewhat to take furtheradvantage of the resonant gap-andpost structure. As shown in Fig. 3,where corresponding numbers are used to denote corresponding parts, thismay be formed as an entity, whose geometry can be accurately establishedwithout any obstruction from the waveguide, the posts in this assemblyserving to fix the relative orientation of the gap electrodes evenbefore the assembly is inserted and united to the waveguide using flange46 as a guide that seats in a recess in the waveguide provided for it.This control over the geometry of the resonant gap and iris structure,made possible by utilizing inductive posts in place of baffles, is afurther advantage attained by the invention.

Additional changes in matters of detail and varied application of thevarious aspects of the invention will readily occur to those skilled inthe art in the light of this disclosure; and accordingly the appendedclaims should be accorded that broad latitude of interpretation as isconsistent with the spirit and scope of the invention.

What is claimed is:

1. A broad-band transmit-receive tube, including a length of rectangularwaveguide having signal-transmitting resonant windows at the endsthereof, said waveguide being hermetically sealed and containing amixture of ionizing and quenching gases at low pressure, multiple pairsof discharge gap electrodes, the electrodes of each pair extendingtoward each other from the opposed broad walls of the rectangularwaveguide and having flanges sealed to the waveguide, said pairs ofdischarge gap electrodes being spaced from each other and from thewindows by quarter-wave intervals, and a post at each side of each pairof discharge gap electrodes extending from one of the opposed broadwalls of the rectangular waveguide to the other at the flanges of thedischarge gap electrodes and resonant with the adjacent discharge gapelectrodes at a frequency within the operating band of the tube.

2. A gaseous discharge tube, including a length of waveguide providing amicro-wave transmission path, and multiple resonant discharge gapassemblies distributed along the waveguide at quarter-wave separation,said assemblies each including a pair of discharge gap electrodes and apair of posts contacting the waveguide only at the base of eachdischarge gap electrode, said posts being readily deformable foradjustment of their inductive effect.

3. A micro-wave device including a length of waveguide providing awaveguide path, a pair of electrodes therein extending toward but spacedfrom each other, and wires deformable for adjustment of their inductiveetfect disposed at opposite sides of said electrodes and extending fromthe base of each of said electrodes to the base of the other, theelectrodes and wires constituting a resonant system.

4. A micro-wave device, including a gas-filled envelope, a pair ofdischarge gap electrodes disposed therein and extending toward eachother, and a pair of wires deformable for adjustment of their inductiveeffect and joined to both said electrodes at the opposite extremitiesthereof.

5. A micro-wave device including an assembled pair of discharge gaelectrodes having gap portions close to each other and base portionsremote from each other, and a pair of wires laterally separated fromeach other and from the gap portions, said wires being deformable foradjustment of their inductive effect and extending from the base portionof each electrode to the base portion of the other.

6. A gaseous discharge tube, including a length of waveguide proyj dinga micro-wave transmission path,

and multiple resonant discharge gap assemblies distributed along theWaveguide at quarter-wave separation, said assemblies each including apair of discharge gap electrodes mounted at opposite sides of thewaveguide and a pair of posts deformable for adjustment of theirinductive effect extending from one of said sides of the waveguide tothe other of said sides of the waveguide and contacting said sides onlyat the base of each discharge gap electrode, said posts being resonantwith said gap electrodes at an operating frequency of the tube.

References Cited in the file of this patent UNITED STATES PATENTSSpencer Sept. 24, 1946 Rochester Dec. 17, 1946 Fiske Feb. 7, 1950 ScottNov. 11, 1952 Hunter June 30, 1953 Booth June 1, 1954

